1. Field of the Invention
The present invention is related to liquid immersion lithography, and more particularly, to efficient recirculation of liquid in immersion lithography systems.
2. Description of the Related Art
An integrated circuit (“IC”) integrates a large number of electronic circuit elements, including transistors. The circuit elements are manufactured and interconnected on a semiconductor substrate, e.g., on a single crystalline silicon wafer. During manufacturing, the wafers undergo cycles of film deposition and lithography, in addition to other processing. Film deposition is the process of depositing a layer of material, e.g., insulating or metallic, over the entire substrate; lithography is the process of patterning the deposited layer. The first step in lithography involves coating the wafer with photoresist that is sensitive to particular radiation, typically ultra-violet light. During the next step—exposure—the substrate is exposed to a radiation pattern stored on a mask, also called a reticle. Radiation locally changes the physical or chemical properties of the photoresist, and the exposed (or unexposed) areas are selectively dissolved during a developing step that leaves behind a pattern of photoresist. The patterned photoresist provides a pattern for a subsequent etching step. The etching step removes undesired areas of the deposited layer, leaving behind structures associated with circuit elements, such as wires, resistors and transistors, and the like.
Highly integrated circuits require small circuit elements. Since the radiation pattern shapes the circuit elements, the smallest feature size depends on the resolution achieved in the lithography exposure step, or the resolution of the projection device used to project the radiation pattern onto the substrate.
According to the Raleigh criterion, this resolution is proportional to the wavelength λ of the projected light and to an adjustment factor k1, and inversely proportional to the sine function of the marginal, or capture, angle θ of the projection optics, where:Resolution =k1*λ/sin(θ)
The resolution can be decreased, i.e., improved, in one of three ways. First, the wavelength λ of the projected light can be decreased. A shorter wavelength, however, may require new photoresist and a number of changes in the projection device, such as using a different light source and light filters, and special lenses for the projection optics. Second, the resolution can be decreased by decreasing the adjustment factor kl. Decreasing k1 may also require the use of different photoresist and high precision tools. Third, the marginal angle θ can be increased by increasing the size of the projection optics. The effect of this increase, however, is limited by the sine function above.
One way to reduce the wavelength λ of the projected light is through the use of immersion lithography, where a liquid is injected between the projection optics and the wafer, taking advantage of the higher refractive index of the liquid compared to air (and, therefore, resulting in a smaller effective wavelength λ).
One of the persistent problems in immersion lithography relates to ensuring purity and lack of contamination of the immersion liquid. The immersion liquid is generally recirculated, using some form of an injection system to inject the liquid into the volume between the projection optics and the substrate, and extracted using some form of a extraction, or suction system to extract the liquid from the exposure area back into recirculation. However, the liquid can get contaminated, for example, due to pickup of particles from the air, or due to pickup of material from the photo resist that is being exposed. Normally, filtering systems are in place to remove the contaminants. However, not all of the liquid that is injected into the exposure area can actually be recirculated. This is due to the surface tension that exists between the liquid and the substrate surface. Although most of the liquid can be extracted, using the suction pressure of the extraction/recirculation system, some droplets of liquid remain on the surface of the way of the substrate, together with their contaminants. Increasing the suction pressure generally does not help past a certain point, since this will increase the recirculation speed, but will not address the problems caused by the surface tension of the liquid.
Even with the current designs, many liquid injection and extraction systems utilize a fairly complex showerhead design to deliver and extract the immersion liquid. However, even in such complex designs, the problem of the surface tension of the liquid is not entirely solved.
Accordingly, what is needed is an approach that ensures that all of the injected liquid is collected by the extraction system in an immersion lithography tool.